Axial resolution is determined by the ______.

• A. Spatial pulse length
• B. Frame rate
• C. Propagation speed
• D. Lateral beam width

Answer: (A)

Axial resolution is determined by spatial pulse length. This is the distance over which one ultrasound pulse extends along the beam path, so it governs how closely two structures aligned along the beam can be distinguished. A shorter spatial pulse length—achieved by higher frequency and/or fewer cycles per pulse—reduces the overlap of echoes from adjacent interfaces, making them appear as separate echoes in time and thus improving axial resolution. The other factors don’t set axial resolution: frame rate affects how many frames per second are shown (temporal resolution), propagation speed affects how we convert travel time to depth, and lateral beam width governs lateral resolution, not along-beam (axial) detail.